S.U.N. is the acronym for (Session Up Nigga)
-it is a term used when you want to have a smoking session but don't want to say what you are doing. instead one can just use the term S.U.N
Bob was at work and wanted to catch George a Buzz but didn't want his boss to know so he looked at george and with a smile said S.U.N. (session up nigga) before walking to his car.